The invention relates to a gas generator on a pyrotechnical basis for the generation of cooled or cooled and filtered gases, preferably for the use in airbag systems, particularly for the use in side airbag systems.
In the case of airbag systems, gas generators of this kind serve for filling the airbag, which then protects the passenger in a collision. Furthermore, such gas generators can be used for filling other devices that require very short reaction and/or filling times.
The ignition unit of the gas generator is ignited by way of a current pulse and then ignites the pyrotechnical gas generating mixture, which, as a rule, burns off under high pressure and provides the required amount of gas. As the combustion of all pyrotechnical mixtures known so far produces not only high-temperature gases but mostly also solid components, it is necessary to cool or cool and filter the combustion gases.
In order to solve this problem, a large number of cooling and filtering arrangements are known, all of which, however, are arranged axially, radially, or axially and radially. DE 196 17 249, for example, describes an axial and radial arrangement of these cooling and filtering elements. These known arrangements of the cooling and filtering systems show the following disadvantages.
If the cooling and filtering system is arranged axially, only a relatively small flow-through cross-section is available, which results in insufficient cooling and bad filtering. In the case of gas generators with small diameters and accordingly great length, this arrangement is disadvantageous because the required free cross-section for the gas flow becomes very small.
If the cooling and filtering system is arranged radially, the problem arises that if the cooling and filtering system is in direct contact with the inner wall of the gas generator housing it is essentially effective only in the area of the exit port, which results in the known problem as described in DE 196 17 249, i. e., the filter material in the area of the exit ports is highly strained. In extreme cases, the filter material is punched or hole patterns may occur therein. In such cases, the gas generated in the combustion chamber can almost freely exit the combustion chamber.
In a spaced arrangement of the radial cooling and filtering system, as described according to the invention of DE 196 17 249, a lot of space is needed. This disadvantage increases as the diameter of the generator decreases, which mandates an increase in the corresponding length.
However, particularly in the case of gas generators for side airbag systems, tubular gas generators of small diameter and correspondingly high length are preferred for reasons of installation.
Thus, it is an object of the present invention to achieve a cooling system, or cooling and filtering system, which enables the design of a gas generator that avoids the aforementioned disadvantages and offers an optimum arrangement of the cooling system, or cooling and filtering system, as regards the space required and the mode of operation.
This object is achieved according to the invention in a surprisingly simple manner in that the cooling system, or cooling and filtering system, tapers, in particular conically, in the discharge direction. Thus, the inner space of the gas generator is separated into two chambers, the combustion chamber containing the gas generating mixture, which is usually present in the form of tablets, but can also be present in the form of granules or the like, and the outflow chamber serving as cooling-off zone for the combustion gases. In the outflow chamber, other deflagration products can be filtered and condensed, too.
The cooling system, or cooling and filtering system, arrangement which, according to the invention, tapers in the discharge direction, in particular conically, the larger cross-section being on the side of the ignitor and the smaller cross-section being on the side of the exit port, serves for achieving that with increasing gas flow in the direction of the exit port, wherein the cooling-off zone gets larger and thus a correspondingly larger outflow chamber is available for the increasing mass flow. Thus, a more constant strain on the cooling system or cooling and filtering system is achieved, too. The cross-section may have a circular, elliptical, square or polygonal shape.
Optionally, the cooling system, or cooling and filtering system, may have 2 or more elements on its outer side that maintain a defined distance and a centered arrangement relative to the inner wall of the gas generator.
Whether only a cooling system or a cooling and filtering system is to be used for the, particularly conical, arrangement according to the invention, which tapers in the discharge direction, depends on the type of the gas generating mixture and the requirement with regard to the admissible amount of particles.
Preferably, the cooling system consists of a grid, a perforated plate, a texture or a sintered material. Depending on the porosity of these materials, apart from the cooling effect, a certain filtering effect is achieved. A further reduction of the particles from the gas flow takes place in the outflow chamber between the cooling system and the inner wall of the generator because particles deposit on the inner wall of the generator, and liquid combustion products that may be present condense on the inner wall of the generator. These effects are supported by the, particularly conical, cooling system, which according to the invention, tapers in the discharge direction. Thus, it is achieved that, with increasing mass flow of the combustion gases in the direction of the exit ports, the flow rate will not increase correspondingly because the increasingly free cross-section in the direction of the exit ports reduces the flow rate.
The cooling and filtering system is either formed by using materials such as grids, perforated plates or texture with small pore size, or sandwich arrangements and multi-layered structures may be employed. Sandwich arrangements of this kind consist of several layers of the same or different materials with the same or different pore sizes. In one embodiment, the cooling system, or cooling and filtering system, has a multi-layered design with a coarse-pored structure (e.g. aperture blind) on the inside and a fine-pored structure (e.g., texture) on the outside. In another embodiment, the cooling system, or cooling and filtering system, comprises a multi-layered design with a fine-pored structure on the inside and a coarse-pored structure on the outside. Here, the term xe2x80x9cinsidexe2x80x9d designates the area facing the combustion chamber. The term xe2x80x9coutsidexe2x80x9d designates the area facing the inner wall of the gas generator.
All of the above described advantages of the, particularly conical, arrangement which, according to the invention, tapers in the discharge direction apply, of course, to cooling and filtering systems in sandwich designs of this kind.